cryptohome: Remove CryptohomeLibraryStubImpl

This production/non-production switching layer is unnecessary, as
CryptohomeClient provides such a layer already.

BUG=305441
TEST=none
R=hashimoto@chromium.org, pneubeck@chromium.org

Review URL: https://codereview.chromium.org/26652002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227880 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227880